Catalytic converters are commonly made from ceramics containing a large proportion of synthetic cordierite. The manufacturing process deliberately aligns the cordierite crystals to make use of the very low thermal expansion along one axis. This prevents thermal shock cracking from taking place when the catalytic converter is used.[6]

As the transparent variety iolite, it is often used as a gemstone. The name "iolite" comes from the Greek word for violet. Another old name is dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong pleochroism. It has also been called "water-sapphire" and "Vikings' Compass" because of its usefulness in determining the direction of the sun on overcast days, the Vikings having used it for this purpose.[7] This works by determining the direction of polarization of the sky overhead. Light scattered by air molecules is polarized, and the direction of the polarization is at right angles to a line to the sun, even when the sun's disk itself is obscured by dense fog or lies just below the horizon.[8]

Gem quality iolite varies in color from sapphire blue to blue violet to yellowish gray to light blue as the light angle changes. Iolite is sometimes used as an inexpensive substitute for sapphire. It is much softer than sapphires and is abundantly found in Australia (Northern Territory), Brazil, Burma, Canada (Yellowknife area of the Northwest Territories), India, Madagascar, Namibia, Sri Lanka, Tanzania and the United States (Connecticut). The largest iolite crystal found weighed more than 24,000 carats, and was discovered in Wyoming, US.[9]

Another name for blue iolite is steinheilite, after Fabian Steinheil, the Russian military governor of Finland who observed that it was a different mineral from quartz.[10] Praseolite is another iolite variety which results from heat treatment. It should not be confused with prasiolite.[11]

^Guillot, Agnès; Meyer, Jean-Arcady (2010) [Published in French in 2008]. How To Catch a Robot Rat: When Biology Inspires Innovation [La bionique: Quand la science imite la Nature]. Translated by Susan Emanuel. The MIT Press. p. 212. ISBN978-0-262-01452-6. Many insects and a few birds perceive polarized light. The Vikings used cordierite for this purpose, a stone that allowed them to reckon the position of the sun by observing the stone's changes in color.